1. Field of the Invention
The present invention relates to an active matrix display device having a thin-film transistor disposed for each pixel.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter referred as an xe2x80x9cLCDxe2x80x9d) can be made thin, compact, and lightweight, and has low power consumption. Liquid crystal displays are now used as display devices in various types of electronic equipment such as portable information equipment. LCDs which have a thin-film transistor disposed as a switch element for each pixel are referred to as active matrix type LCDs, and such panels are used as high resolution, high display quality display devices because they can reliably maintain the display content of each pixel.
FIG. 1 shows an equivalent circuit of a pixel of an active matrix LCD. Each pixel is provided with a thin-film transistor (TFT) 1 which is connected to a gate line and a data line and, when the TFT is turned on by a selection signal output to the gate line, data corresponding to the display content is supplied from the data line to a liquid crystal capacitor 2 (Clc) through the TFT. Here, a storage capacitor 3 (Csc) is connected to the TFT in parallel to the liquid crystal capacitor Clc because it is necessary to securely maintain the written display data for a period during which the TFT is selected, data is written and the next TFT is selected.
FIG. 2 shows a plane structure of a pixel section of the TFT formed substrate (a first substrate 100) of a conventional LCD, and FIG. 3 shows a sectional structure of the LCD taken along line Xxe2x80x94X of FIG. 2. The LCD has a structure in which liquid crystal is sealed between first and second substrates. In this active matrix LCD, TFTs 1, pixel electrodes 74, etc. are arranged in a matrix on the first substrate 100, and a common electrode 56 to which a common voltage Vcom is applied, a color filter 54 and the like are formed on the second substrate disposed to face the first substrate. The liquid crystal capacitor Clc is driven for each pixel by a voltage applied between the respective pixel electrodes 74 and the opposing common electrode 56, with liquid crystal 200 between them.
The TFTs disposed for each pixel on the side of the first electrode 100 are so-called top gate TFTs which have a gate electrode 60 disposed on a layer above an active layer 64, as shown in FIG. 3. The active layer 64 of the TFT is patterned on a substrate 5 as shown in FIG. 2, a gate insulating film 66 is formed to cover the active layer 64, and the gate line, which also serves as the gate electrode 60, is formed on the gate insulating film 66. The active layer 64 has a channel region 64c positioned to face the gate electrode 60, and a drain region 64d and a source region 64s, in which an impurity is charged, are formed on both sides of the channel region 64.
The drain region 64d of the active layer 64 is connected to a drain electrode 70, which also serves as the data line, through a contact hole which is formed in an interlayer insulating film 68 to cover the gate electrode 60.
A planarization insulating film 72 is formed to cover the area above drain electrode and data line 70, and the source region 64s of the active layer 64 is connected to a pixel electrode 74, which is formed on the planarization insulating film 72 of ITO (indium tin oxide) or the like, through the contact hole.
The source region 64s of the active layer 64 also serves as a first electrode 80 of the storage capacitor Csc disposed for each pixel and extends from the contact region with the pixel electrode 74 as shown in FIG. 2. A second electrode 84 of the storage capacitor Csc is simultaneously formed of the same layer as the gate electrode 60 as shown in FIG. 3, in a distinct region separated from that of the gate electrode 60 by a prescribed gap. A dielectric substance between the first electrode 80 and the second electrode 84 is also served by the gate insulating film 66. The second electrode 84 of the storage capacitor Csc, which is not independent for each pixel, extends on the pixel region in the line direction in the same way as the gate line 60 as shown in FIG. 2. To this second electrode 84 is applied a predetermined storage capacitor voltage Vsc.
Thus, a storage capacitor Csc is disposed for each pixel to hold an electric charge corresponding to the display content which must be applied to the liquid crystal capacitor Clc during a TFT non-selection period. As a result, it is made possible to suppress a potential change of the pixel electrode 74 and to maintain the display content.
In applications in which it is required the display device be compact and have a high resolution, the area per pixel must be made small, and, as a consequence, the liquid crystal capacitor Clc per pixel becomes small. Therefore, a storage capacitor Csc such as described above must be provided to ensure that the display data of each pixel is maintained during the unit display period.
However, because the storage capacitor Csc does not function as a display region, reduction of the displayable area per pixel, namely, a reduction in aperture ratio, cannot be avoided in a transparent type LCD. Especially, when the second electrode 84 of the storage capacitor Csc is formed on the same layer as the gate line 60 as shown in FIG. 2 and FIG. 3, an insulating space is required to prevent the gate line 60 and the second electrode 84 from being short-circuited. Furthermore, because the second electrode region is formed of the same material as the gate, it is also opaque. As a consequence, aperture ratio is further lowered accordingly and producing a bright display becomes even more difficult.
Still further, a conventional LCD is provided with a black matrix for shielding light between the pixels in order to improve the contrast of the screen in the region between pixels. This matrix is, typically provided, in addition to the above storage capacitor Csc, on the second substrate which is disposed to face the first substrate on which the TFT is formed. The LCD is formed by bonding the first substrate and the second substrate and sealing liquid crystal in the gap between them as described above, and, in order to avoid variations in the aperture ratio of the respective pixels caused by displacement of the bonded substrates, either the black matrix is made to have a larger width or small pixel regions (e.g., pixel electrode) are formed. This further exacerbates the problem of the aperture ratio.
To address the above problems, it is an object of the present invention to provide an active matrix display device which simultaneously provides adequate storage capacitance and a high aperture ratio.
In order to achieve the aforementioned object, the present invention is directed to an active matrix display device having a thin-film transistor (TFT) and a storage capacitor in respective pixels, wherein the TFT is formed on a substrate as a top gate type for each pixel; a first electrode of the storage capacitor is electrically connected to an active layer of the TFT; and a second electrode of the storage capacitor is formed to partly overlap at least the active layer of the TFT with an insulating layer provided between the active layer and the substrate.
Another aspect of the invention is directed to an active matrix display device which has a TFT, a liquid crystal capacitor and a storage capacitor in respective pixels and drives liquid crystal sealed in a gap between first and second substrates to display data, wherein the TFT is formed on the liquid crystal-opposed side of the first substrate as a top gate type and the storage capacitor is formed in a region formed between a first electrode which is also served by an active layer of the TFT and a second electrode which is disposed with an insulating film held between the active layer of the TFT and the first substrate.
As described above, the first electrode of the storage capacitor is connected to, or serves as, the active layer of the TFT, and the second electrode is disposed, not on the same layer as a gate line, but, rather, below the first electrode. Therefore, a storage capacitor Csc having a sufficient size can be formed for each pixel without lowering the aperture ratio.
In the active matrix display device according to another aspect of the invention, the second electrode of the storage capacitor is provided with a light-shielding function.
In the active matrix display device according to still another aspect of the invention, the second electrode of the storage capacitor is formed of a light-shielding material in a region excluding a pixel aperture region.
In the active matrix display device according to still another aspect of the invention, the second electrode of the storage capacitor is formed in a region excluding a pixel aperture region and also serves as a black matrix.
When the second electrode of the storage capacitor disposed below the active layer of the TFT is lightproof, an optical leak current can be prevented from being generated due to outside light incident from the lower position of the active layer. A black matrix preventing the occurrence of an optical leak current in the TFT can also enhance the display contrast.
Using the second electrode as the black matrix can additionally enhance the contrast without lowering the aperture ratio.
In the above active matrix display devices, a polycrystallized polysilicon layer can be made the active layer of the TFT by laser irradiation of a formed amorphous silicon layer.
When the second electrode layer is uniformly formed on the active layer region of the amorphous silicon layer, and particularly below the TFT channel region, annealing conditions for the channel region are uniform during laser annealing for polycrystallization. Therefore, the polysilicon layer has a uniform grain size, and variations in the properties among the TFTs can be prevented.
Another aspect of the invention is directed to an active matrix display device, wherein each of pixels disposed in a matrix is configured in the vicinity of an intersection of a gate line and a data line and provided with a thin-film transistor, a display element, and a storage capacitor; the thin-film transistor is formed on a substrate as a top gate type in each pixel; a first electrode of the storage capacitor is configured with an active layer of the thin-film transistor extended along the data line; and a second electrode of the storage capacitor is formed to overlap with an insulating layer held between the first electrode and the substrate.
As described above, the second electrode of the storage capacitor is arranged on a layer different from that of the gate line and below the first electrode extended from the active layer of the TFT. Thus, it is not necessary to provide a significant insulating space between the second electrode and the gate line, and a region where the first electrode integral with the active layer and the second electrode are overlapped can be efficiently made larger. The region along the data line is mostly a non-display region, and, when the storage capacitor is formed by arranging the first electrode on such a region, a large capacitor can be obtained easily and without lowering the aperture ratio. When the first electrode and the data line which are positioned vertically with the insulating layer therebetween are arranged so as not to overlap in the same plane, coupling between the data line and the first electrode can be prevented.
Another aspect of the invention is directed to an active matrix display device, wherein each of pixels disposed in a matrix is configured in the vicinity of an intersection of a gate line and a data line and provided with a thin-film transistor, a display element, and a storage capacitor; the thin-film transistor is formed on a substrate as a top gate type in each pixel; a first electrode of the storage capacitor is configured with an active layer of the thin-film transistor extended to a region below the data line; a second electrode of the storage capacitor is formed between the first electrode and the substrate to overlap with the first electrode and an insulating layer held between them; and a conductive shielding layer is formed in the region where the data line and the first electrode of the storage capacitor overlap with an insulating layer held between the data line and the first electrode.
Lowering of the aperture ratio due to the formation of the storage capacitor can be minimized by disposing the first electrode and the second electrode of the storage capacitor below the data line forming region, and coupling between the first electrode and the data line can be prevented by disposing the conductive shield layer between the data line and the first electrode. The storage capacitor can also be configured between the first electrode and the conductive shield layer.
In the active matrix display device according to another aspect of the invention, the conductive shielding layer is also served by the gate line which supplies the thin-film transistors of the pixels of another row with a selection signal.
Thus, when the conductive shield layer is served by the gate line of the next stage, a storage capacitor which is not affected by a data line voltage can be configured below the data line without increasing the steps. Because the gate line also serves as the conductive shield layer, it is not necessary to consider an allowance or the layout for securing the insulation between the gate line and the conductive shield layer, and the conductive shield layer can be formed in a minimum space.
Another aspect of the invention is directed to an active matrix display device, wherein each of pixels disposed in a matrix is configured in the vicinity of an intersection of a gate line and a data line and provided with a thin-film transistor, a display element, and a storage capacitor; the thin-film transistor is formed on a substrate as a top gate type in each pixel; a first electrode of the storage capacitor is formed of a semiconductor layer which configures an active layer of the thin-film transistor; a second electrode of the storage capacitor is formed between the first electrode and the substrate to overlap with the first electrode and an insulating layer held between them; and the second electrode is provided with a black matrix function for shielding light between the respective pixels and has at least a channel region of the thin-film transistor of the respective pixels opened.
As described above, according to the present invention, the second electrode of the storage capacitor is arranged on a layer different from that of the gate line and below (on the substrate side) the first electrode formed of a semiconductor layer which configures the active layer of the TFT. When the second electrode is functioned as the black matrix, decrease of the aperture ratio of each pixel due to the displacement of the two bonded substrates can be prevented to a greater extent than when the black matrix is formed on another substrate or the like. The storage capacitor can be efficiently formed in the pixel and the aperture ratio can be improved with a sufficient capacitor secured without necessity of considering a sufficient insulating space between the second electrode and the gate line. Furthermore, the second electrode is open in the channel region of at least the thin-film transistor, so that, when the amorphous semiconductor layer such as an amorphous silicon layer to be described later is polycrystallized by laser annealing so to be used as the active layer, it is not necessary to adjust the annealing conditions of the channel region which largely affect the thin-film transistor characteristics in compliance with the properties when the second electrode is present on the lower layer. Even when the driver section which is provided with the same thin-film transistor as the pixel section is built in the periphery of the substrate, the second electrode is open in the channel region of the thin-film transistor of the pixel section, so that the thin-film transistor of the driver section and the above thin-film transistor of the pixel section can be formed under the same conditions.
In the active matrix display device according to another aspect of the invention, a light shielding layer is formed above a non-opposing side of the active layer to the second electrode at least in the channel region of the thin-film transistor having the second electrode which also serves as the open black matrix protecting the channel region from light.
In the active matrix display device according to still another aspect of the invention, the light-shielding layer is also served by the data line.
Because the vicinity of the channel region having the second electrode open is shielded from light by another shielding layer, lowering of the contrast of the image light due to a leak of light in the vicinity of this channel region can be reliably prevented. Furthermore, because the channel region of the active layer is shielded from light, the occurrence of light leakage on the transistor due to light irradiated to the channel region of each thin-film transistor can also be prevented. Because the data line serves as the light-shielding layer, this region can be shielded from light without adding a special step.
In the active matrix display device according to another aspect of the invention, a polysilicon layer which is polycrystallized by laser irradiation to a formed amorphous silicon layer is used for the active layer of the thin-film transistor.
When laser-annealing for polycrystallization, a difference in thermal capacity in the active layer region of the amorphous silicon layer, and particularly below the TFT channel region, results in a difference in grain size. However the annealing conditions for the channel region can be made uniform by opening the second electrode in the channel region of each TFT. Therefore, the polysilicon layer has a uniform particle diameter, and variations in the properties among the TFTs can be prevented.
Another aspect of the invention is directed to a driver built-in type active matrix display device, wherein a pixel section and a driver section are disposed on the same substrate; the pixel section is provided with a plurality of pixels arranged, and each pixel has a pixel section thin-film transistor, a display element and a storage capacitor; the pixel section thin-film transistor is formed as a top gate type transistor on the substrate of each pixel; a first electrode of the storage capacitor is electrically connected to an active layer of the pixel section thin-film transistor; a second electrode of the storage capacitor is formed to partly overlap at least the active layer of the pixel section thin-film transistor with an insulating layer held between the active layer and the substrate; the driver section has a plurality of driver section thin-film transistors which output a signal for driving the respective pixels of the pixel section; the driver section thin-film transistor is configured as a top gate type transistor on the substrate; an active layer of the driver section thin-film transistor is configured of the same material layer as the active layer of the pixel section thin-film transistor; and a conductive layer which is formed of the same material as the second electrode is disposed between the active layer of the driver section thin-film transistor and the substrate with the insulating layer held between them.
Another aspect of the invention is directed to a driver built-in type active matrix display device which drives liquid crystal sealed in a gap between first and second substrates to display data, wherein a pixel section and a driver section are disposed on the same substrate; the pixel section is provided with a plurality of pixels, each of which has a pixel section thin-film transistor, a liquid crystal capacitor and a storage capacitor; the pixel section thin-film transistor is formed as a top gate type transistor for each pixel on the liquid crystal opposing side of the first substrate; the storage capacitor is formed in a region formed between a first electrode which is also served by an active layer of the pixel section thin-film transistor, and a second electrode which is disposed to oppose the first electrode with an insulating layer held between them and also disposed between the active layer of the pixel section thin-film transistor and the substrate; the driver section has a plurality of driver section thin-film transistors which output a signal for driving each pixel of the pixel section; and the driver section thin-film transistor is configured as a top gate type transistor on the substrate, an active layer of the driver section thin-film transistor is configured of the same material layer as the active layer of the pixel section thin-film transistor, and a conductive layer which is formed of the same material as the second electrode is disposed between the active layer of the driver section thin-film transistor and the substrate with the insulating layer held between the active layer and the conductive layer.
As described above, the first electrode of the storage capacitor is connected to (or serves as) the active layer of the thin-film transistor, and the second electrode is disposed, not on the same layer as the gate line, but below (on the substrate side) the first electrode, so that a storage capacitor Csc having an adequate size can be formed on each pixel without lowering the aperture ratio. The conductive layer which is formed of the same material as the second electrode of the storage capacitor is also formed below (the substrate side) the active layer with respect to the thin-film transistor of the driver section having the active layer which is made of the same material as the active layer of the TFT of the pixel section on the same substrate. Therefore, the conditions for forming the same material layer which configures the active layer of the thin-film transistor of the pixel section and the active layer of the thin-film transistor of the driver section become the same between both transistors, and it becomes possible to produce transistors having identical properties.
In any of the above driver built-in type active matrix display devices according to another aspect of the invention, a polysilicon layer which is polycrystallized by laser irradiation to a formed amorphous silicon layer is used for the pixel section and the active layer of the driver section thin-film transistor.
When the amorphous silicon layer is polycrystallized by laser annealing, the polysilicon layer, which is finally obtained under the conditions such as thermal conductivity in a region where the silicon layer is formed, has a varying particle diameter. When the conductive layer is disposed similarly below (on the substrate side) the active layer of both of the thin-film transistors of the pixel section and the driver section as in the present invention, the particle diameter of the polysilicon layer formed by laser annealing can be prevented from becoming different between the active layers of both transistors, and the transistors having the identical properties can be formed.
In the driver built-in type active matrix display device according to another aspect of the invention, the plurality of driver section thin-film transistors have an n-type channel transistor and a p-type channel transistor which are of different conduction types, and the conductive layer formed between the active layer and the substrate of the n-type channel transistor and the conductive layer formed between the active layer and the substrate of the p-type channel transistor are controlled.
In the thin-film transistor of the driver section which is configured as a top gate transistor, an effect on the transistor caused by the potential of the conductive layer existing below the active layer differs depending on whether the conductive type of the transistor is a p type or an n type. Therefore, a leakage current due to the occurrence of a back channel can be prevented by ensuring an appropriate potential by respectively controlling the potential of the conductive layer disposed below (on the substrate side) the active layer of the thin-film transistor of the driver section with respect to the p-type and n-type transistors according to the present invention.